White LEDs (“WLEDs”) have gained significant importance in the applications of general illumination market and display market. One example is the WLED street lamp application. In another example, traditional cold cathode fluorescent (“CCFL”) backlight is being replaced by LED backlight in the liquid crystal display (“LCD”) TV market. In such applications, a large number of LEDs can be coupled in series as a LED string to provide a desired brightness. The LED string can be driven by a voltage supply, for example, as high as 200V. Multiple strings are further configured to offer the desired backlight. The serially connected LEDs have a uniform current and have less power consumption than other configurations. However, if any LED in a string is damaged and becomes open, the whole string is off.
FIG. 1 schematically shows a traditional solution to bypass an open circuited LED by using a Zener diode. Several LEDs are coupled in series as a LED string and a power supply voltage VSUP (a differential voltage between SUP+ and SUP−) is used to provide power across the LED string. Zener diode triggered snapback transistor ZD is placed in parallel with each LED. Zener diode ZD has a breakdown voltage higher than a normal forward voltage VA0 of the LED. Thereby in normal status of the LED, Zener diode ZD is open and does not consume power. If one LED in the string becomes open, supply voltage VSUP builds up across the open LED, and eventually breaks down the corresponding Zener diode ZD to conduct. Once the Zener diode ZD conducts, it triggers a snapback and clamps a forward voltage VA of the open LED at a clamping voltage VCP of Zener diode ZD.
However, the power consumption of Zener diode is not low and the Zener diode ZD cannot recover from snapbacks when the open circuited condition is removed, unless the entire LED string is rebooted.